The present invention relates to a method for forming a zinc phosphate film on a metal surface.
The zinc phosphate film on a metal surface has been used, for example, in a non-coating product as a surface-protecting film or as a lubricating film while being processed, or in a coating product as an under coating film while being coated.
Hitherto, a zinc phosphate film of this kind can be formed by chemical conversion of a metal surface with an acidic aqueous solution of zinc phosphate and thus, various kinds of methods for forming the zinc phosphate film have been proposed and actually used.
As a method for forming a zinc phosphate film in a non-coating product, there has been known a method wherein an aqueous solution of zinc phosphate added with a calcium ion is used and a fine crystalline zinc phosphate film modified with calcium is formed (Japanese Official Patent Provisional Publication, showa 55-58376). This film can be used as an auxiliary lubricating film when an iron and steel material is treated with cold working.
As a method for forming a zinc phosphate film in a coating product, there have been known methods wherein an aqueous solution of zinc phosphate added with an organic acid such as lactic acid and tartaric acid is used to form a zinc phosphate film on a galvanized steel plate surface (Japanese Official Patent Gazette, showa 54-24973), wherein an aqueous solution of zinc phosphate added with a nickel ion or a manganese ion is used to form a zinc phosphate film (Japanese Official Patent Gazette, showa 61-36588), and wherein a steel plate surface is dipped, before chemical conversion into a zinc phosphate film, in a surface-conditioning solution containing titanium phosphate colloid and then, the chemical conversion forms a fine crystalline zinc phosphate film (Japanese Official Patent Gazette, showa 58-55229).
In said hitherto-known methods, an attempt for making a zinc phosphate film of fine-grained crystals leads to elevation of film-quality, but it is not possible to increase film weight (film thickness) per unit area of the metal surface. The increase of film weight and transformation into fine-grained crystals are an anti-parallel relation.
However, to increase the film weight, particularly, in the non-coating products, it is very important to secure a necessary protecting function and a lubricating function. Needless to say, the increase of film weight should not accompany deterioration of the film quality.